A laminated printed circuit board or multichip module serves as a support substrate for electronic components, such as ICs, resistors, capacitors, inductors, and other components. The laminated printed circuit board (PCB) or multichip module also provides conductive traces for conveying electric current to the terminals of the electronic components. In multilayer constructs each layer may contain circuitry. The layers are electrically connected by means of conductive vertical interconnects (or vias) within the multilayer structure or by electrically conductive vias (through holes) which pass through the entire substrate.
Discrete passive electronic components such as capacitors and resistors which are attached to the printed circuit board generally take up to 60% of the printed circuit board real estate, limiting the space available for active components (e.g. ICs). Therefore, removal of passive components from the PCB surface and their integration into a multilayer board could provide for further miniaturization and increased computing power of the electronic devices. Other benefits are improved environmental stability and reduced system noise and noise sensitivity due to shortened leads. Prime candidates for embedded passive components are decoupling or bypass capacitors as well as termination resistors. Particularly at high frequencies, these functions are often difficult or impossible to perform by passive through-hole or surface-mount components located on the board surface.
In contrast to ceramic thick film and Si-wafer based thin film multichip modules, an efficient embedded passives technology for laminated multilayer boards does not presently exist. This is due to incompatibility of the current materials and processes with printed circuit board fabrication. However, since fabrication of laminated, multilayer PCBs is comparatively inexpensive, the availability of such a technology for PCBs would be very desirable. The invention addresses this need and describes a method of fabricating individual, embedded capacitors for multilayer printed circuit boards.
The concept of embedded capacitance has been to some limited extent addressed earlier in the example of "capacitive planes" (U.S. Pat. Nos. 4,908,258, 5,010,641, 5,261,153) providing a non-individual or "shared" capacitance. Capacitive planes consist of two laminated metal sheets insulated by a polymer based dielectric layer. Shared capacitance, as the name implies, requires the timed use of the capacitance by other components. In contrast, the invention detailed here permits the construction of individual, embedded capacitors anywhere inside or on the top surface of a laminated multilayer board and therefore offers high design flexibility. The critical element of the invention is the use of a patternable insulator which is patterned to define thickness and area of a capacitor dielectric.